In conventional methods extensively used for AC motor driving apparatus, motor current is detected so that motor toque is controlled with high precision. For cost reduction and other purposes, however, it is desirable to reduce the number of sensors that detect motor current. There are some known examples of methods of reducing the number of sensors.
In Japanese patent laid-open No. Hei 6 (1994)-153526, a current sensor is provided on a DC line of a pulse width modulation (PWM) inverter. Three-phase currents output from the inverter are estimated from the switching pattern of the inverter and a detected DC current value.
In Japanese patent laid-open No. 2004-297966, a current sensor is provided on a DC line of a PWM inverter and a position detector is also provided that detects the rotor phase of an AC motor. The rotor phase is used to convert a detected DC current value into rotational coordinates.
In Japanese patent laid-open No. 2004-274841, a current sensor is provided on a particular phase of an AC motor. The rotor phase of the AC motor is detected from the pulsation of a detected current value so as to drive the motor.
The rotor position detector of an AC motor used for driving purposes, which is applied to a vehicle or the like, requires highly precise torque control, starting from the state at which the motor is stopping. Therefore, a current sensor is used to detect the motor current of each phase. It is known that a so-called one-pulse mode is used, in which PWM is overmodulation and a rectangular voltage for one pulse is applied in each half a cycle so that high-speed rotation of an AC motor is performed more efficiently and the voltage of an inverter power supply is used more efficiently.